Method of making porous metal elements



Patented May 9, 1939 UNITED STATES PATENT cl rics,

METHOD oll wfilagmns METAL James B. Davis, Dayton, dhio, assignor to.General Motors Corporation, Detroit, Mich, a corporation of Delaware NoDrawing. Application June 18, 1936,

Serial No. 85,889

6 Claims. (01. 75-22) This invention relates to porous metallic filterelements.

An object of this invention is to provide a thin finely porous metallicsheet which is adapted for use as a filter element. Such filter elementsmay be used for removing extremely fine solid particles from a liquid,for instance for clarifying cofiee by removing the very fine cofieeparticles therefrom. Y

Heretofore filter elements for clarifying a liquid of dust-likeparticles have been made of nonmetallic materials such as matted orwoven fibers such as paper or textile fabrics,'or porous skins. Afterthe pores of such non-metallic filter elements have become more or lessclogged with solid particles it is dimcult to remove such solidparticles due to the relative weakness and easy destructibility of suchnon-metallic filter elements and hence they are usually discarded afteronly a limited period of use. The metallic filter elements of thisinvention may be cleansed by burning the solid particles clogging sameby means of a hot blast at a temperature which will not destroy theporous metal structure. Also the metallicv filter elements of thisinvention may be cleansed of some kinds of non-metallic foreignparticles, by the chemical action at a suitable temperature of chosengases or liquids which will attack the non-metallic particles but willnot attack the metal structure of the filter element. In these respectsa metallic filter elementmade according to this invention has distinctadvantages-over previously known non-metallic filters.

Further advantageslof the filter elements of this invention over knownflexible forms of filter elements are:

(1) Greater strength and rigidity to sustain thepressure of the liquidbeing filtered;

(2) Greater uniformity in degree of porosity or i0 fineness of pores;

(3) Economy of manufacture of a large num-' ber of filter elements allhaving the same degree of porosity;

(4) Economy of manufacture of a metallic L5 filter element of anysuitable metals or metal alloys so that the metal of the filter elementwill not contaminate the liquid being filtered, or will not be corrodedby the liquid being filtered.

Further objects and advantages of the present i0 invention will beapparent from the following description: l

As a specific example of. making a filter element according to thisinvention the following is given:

5 A homogeneous mixture of finely divided nickel and copper powdersisprepared having about 68% nickel and 32% copper therein. The degree offineness of the metal powder determines to some extent the degree ofporosity, of the final filter element, that is, the average size off-the5 pores thru the filter element. The size of thenickel and copperpowders may vary frompowder screened thru a wire mesh screen havingabout openings per linear inch to 300 or more openings per inch. I

. The powder mixture of nickel and copper powders is spread loosely upona hard smooth surface of a graphite mold to form a loose powder layer ofsubstantial depth. The top of the powder is scraped off to a smoothlevel surface by any suitable means to form a loose uncompacted layer ofpowder having the desired depth, for example 3, inch deep. Themoldandmncompacted powder layer thereon is then heated in anon-oxidizing or reducing atmosphere in a suitable furnace at atemperature of 2025 F. for a short time (about 8 minutes) to cause thenickel and copper particles to alloy or sinter togetherto form a highlyporous rigid metal sheet of an alloy havingMonel metal composition andwhich will resist corrosion to a high degree. Afterjthis sintering stepis completed, the porous metal sheet is cooled in a non-oxidizingorreducing atmosphere and then readily lifted from the hard graphitesurface upon which it lay du'ringsintering. This results in a highly'porous metal membrane which'may be used as a filter element having theadvantages described above. If the metal powdersused in this process arequite coarse,"for instance having a coarseness determined by a 40 meshscreen, the final porous membrane-.will-havc correspondingly relativelylarge pores therethrough to more freely permit the passage of liquidtherethru. r If the metal particles are so small as to pass thru a 150mesh screen the final porous membrane will have considerably smallerpores,.and so on for still smaller metalparticle's. Any other desiredproportions of copper and nickel may be used instead of the proportionsdescribed above to form any other desired coppernickel alloys, forinstance 95 parts copper and 5' parts nickel. Also various other metalsor combinations of metals may be used to give the desired metal alloyinthe final porous membrane, in each case the sintering temperature usedbeing-v such as will cause the metal powders being used to alloy orsinter together without such melting as will destroy the high porosityof the final metal structure. In order to provide ahighly porous bronzemembrane any desired proportions of copper and tin metal powders may beused in the process described above. For instance from about 90% toabout 97% copper powder may be used with from 10% to 3% of tin powder.The sintering temperature for copper-tin mixtures is preferably quiteclose to 1500 F.

If desired, the highly porous metal membrane made by the processdescribed above may be somewhat compacted after the sintering step tocontrol its porosity or density. This is preferably done by passing thehighly porous membrane between pressure rolls or by any other suitablecompression method. Obviously the degree of porosity of the highlyporous metal membrane may be reduced as much or as little as desired bysuch rolling of the porous metal structure.

Porous metal membranes having a very high degree of uniformity ofporosity may be rapidly and very economically made by the method of thisinvention. Such membranes may be used in general for filtering foreignmatter from a liquid or from a gas or air where the size of the.

timately mixed finely divided uncompacted readily alloyable metalpowders including at least two constituent metals of different meltingpoints upon a hard smooth mold surface having little or no bondingaffinity for said metal powder when sintered thereupon, heating theuncompacted loose layer of metal powders in a reducing atmosphere at atemperature above the melting point of the lowest melting constituentmetal and for a short time period as will cause the loose metalparticles to alloy together without such melting as to close the voidsbetween the metal particles and thereby form a thin relatively stronghighly'porous metal sheet, then removing the highlyporous metalsheet'from the mold surface.

2. The'steps in the method of making a relatively thin sheet of highlyporousmetal, comprising: distributing intimately mixed finely divideduncompacted readily alloyable metal powders including at least twoconstituent metals if. different melting points upon a mold surface ofgraphite and thereby forming a uniform layer of uncompacted metalpowders, heating the loose layer in a reducing atmosphere at atemperature above the melting point of the lowest melting constituentmetal and for a short time period sufficient to cause the metalparticles thereof to partially fuse or alloy together without suchmelting as to close the voids between the metal particles and therebyforming a thin relatively strong highly porous metal sheet whose poresextend entirely thru said sheet, and subsequently removing said poroussheet from the mold 3 surface;

3. The steps in the method of making a relatively thin sheet of highlyporous alloy material comprising: distributing intimately mixed finelydivided non-compacted readily alloyable metal powders including at leasttwo constituent metals of different melting points upon a hard smoothmold surface having little or no bonding affinity with the metal powderswhen sintered thereupon,

heating the non-compacted loose layer of metal powders undernon-oxidizing conditions at such a temperature above the melting pointof the lowest melting constituent metal and for a short time as willcause the loose metal particles to alloy together without such meltingas to closethe metal powder under non-oxidizing conditions at atemperature above the melting point of the lowest melting componentmetal and below the melting point of the highest melting component metalfor a short time as will cause the loose metal particles to alloytogether without such melting as to close the voids between the metalparticles and thereby forming a relatively strong porous metaflayerhaving intercommunicating pores extending entirely through the layer,and then cooling the sintered layer under non-oxidizing conditions.

5. The steps in the method of making a relatively thin layer of porousalloy material having intercommunicating pores extending entirelythrough the layer, comprising: distributing intimately mixed finelydivided copper and nickel powders in a loose non-compacted layer upon asupporting surface, sintering the non-compacted loose metal powdersunder non-oxidizing conditions at a temperature above the melting pointof copper and below the melting point of the nickel for a short time asto cause the alloying of the copper and nickel particles without suchmelting as to close the voids between the metal particles, and thencooling the sintered metal layer under non-oxidizing conditions.

6. The steps in the method of making a relatively thin layer of porousbronze having intercommunicating pores extending entirely through thelayer, comprising: distributing intimately mixed finely divided metalpowders adapted to form a bronze in a loose non-compacted layer upon asupporting surface, sintering the loose non-compacted layer of metalpowders under non-oxidizing conditions at a" temperature above themelting point of the lowest melting powdered metallic constituent andbelow the melting point of the highest melting constituent for a shorttime as to cause alloying of the metal powders without such melting asto close the voids between bronze layer under non-oxidizing conditions.

JAMES H. DAVIS.

